Organic light-emitting diodes (OLEDs) are attractive devices because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described a multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211. OLEDs that produce white light (white OLEDs) efficiently are considered a low cost alternative for several applications such as paper-thin light sources, backlights in liquid crystal displays (LCDs), automotive dome lights, and office lighting. As with any light-emitting device, it is desirable that white OLEDs be bright and efficient in terms of power consumption. The preferred spectrum and precise color of a white OLED will depend on the application for which it is intended. For example, if a particular application requires light that is to be perceived as white without subsequent processing that alters the color perceived by a viewer, it is desirable that the light emitted by the OLED have 1931 Commission International d'Eclairage (CIE) chromaticity coordinates, (CIEx, CIEy), of about (0.33, 0.33). For other applications, particularly applications in which the light emitted by the OLED is subjected to further processing that alters its perceived color, it can be satisfactory or even desirable for the light that is emitted by the OLED to be off-white, for example bluish white, greenish white, yellowish white, or reddish white. Hereinafter, the term “white” will be used broadly to mean light that is perceived as white or off-white. The CIE coordinates of such light satisfy, at least approximately, the condition that the quantities (CIEx+0.64 CIEy) and (0.64 CIEx−CIEy) be in the range of 0.36 to 0.76 and the range of −0.20 to +0.01, respectively. A white OLED will mean an OLED whose emission is white in this broad sense.
The following patents and publications disclose OLEDs capable of emitting white light, comprising a hole-transporting layer and an organic luminescent layer, and interposed between two electrodes. White OLEDs have been reported before by J. Shi in U.S. Pat. No. 5,683,823, wherein the luminescent layer includes red and blue light-emitting materials uniformly dispersed in a host emitting material. These devices have good electroluminescent characteristics, but the concentrations of the red and blue dopants are very small, such as 0.12% and 0.25% of the host material. These concentrations are difficult to control during large-scale manufacturing. Sato et al., in JP 07,142,169, disclose an OLED capable of emitting white light, made by forming a blue light-emitting layer adjacent to a hole-transporting layer, followed by a green light-emitting layer having a region containing a red fluorescent dye. Kido et al., in Applied Physics Letters Vol., 64, p. 815 (1994), report a white OLED in which a single light-emitting layer contains a polymeric host and three fluorescent dyes emitting in different spectral regions. Kido et al., in Science, Vol. 267, p. 1332 (1995), report another white OLED. In this device, three light-emitting layers with different carrier transport properties, and individually emitting blue, green or red light, are used to generate white light. Littman et al., in U.S. Pat. No. 5,405,709, disclose another white OLED that includes an electron-transporting layer doped with a red dopant and also includes a blue light-emitting recombination layer contiguous with a hole-injecting and hole-transporting zone. Deshpande et al., in Applied Physics Letters, Vol. 75, p. 888 (1999), describe a white OLED using one layer with green luminescence and a second layer with red and blue luminescence, the two layers being separated by a hole blocking layer.
White OLEDs can be used with color filters in full-color display devices. They can also be used with color filters in other multicolor or functional-color display devices. White OLEDs for use in such display devices are easy to manufacture, and they produce reliable white light in each pixel of the displays. However, the color filters each transmit only about 30% of the original white light. Therefore, the white OLEDs must have high luminous yield. Although the OLEDs are referred to as white and can appear white or off-white, for this application, the CIE coordinates of the light emitted by the OLED are less important than the requirement that the spectral components passed by each of the color filters be present with sufficient intensity in that light. It is also important that the color, after passage through a color filter, be appropriate for the intended application. For use in a full-color display, typical desired colors after passage through a red, green, or blue filter are, respectively, red with CIE coordinates of about (0.64, 0.36), green with CIE coordinates of about (0.29, 0.67), and blue with CIE coordinates of about (0.15, 0.19). The devices must also have good stability in long-term operation. That is, as the devices are operated for extended periods of time, the luminance of the devices should decrease as little as possible.
A problem in the application of white OLEDs, when used with color filters, is that the intensity of the red component of the emission spectrum is frequently lower than desired. Therefore, passing the white light from the OLED through the red filter provides red light with a lower efficiency than desired, and the power that is required to provide a desired intensity of red light is higher than desired. Consequently, the power that is required to produce a white color in the display by mixing red, green, and blue light can also be higher than desired.